The actual events are not exactly clear of what happened to lead to the separation of H.kennerlyi from the others. This likely occured during the Pleistocene when glaciers were retreating and advancing causing lots of changes in refugia for many organisms. I believe that H. kennerlyi represent those populations in the souther refugia and H. nasicus represent the northern refugia.
I believe something like this happened in the Pliocene prior to the Pleistocene that caused the split between Heterodon nasicus/kennerlyi and Heterodon simus. You will find many many reptiles that have the same kind of distributions as these two with regards to having a southeastern population and a southcentral population. This includes a species of tortois, many snakes (like coral snakes, rattlesnakes, many colubrids), and lizards. I can't vouch for other groups because of their high mobility meaning that their distributions may be a result of secondary migration to those areas.
So it is more likely that the rising and falling Gulf of Mexico played a larger part in the formation of H. simus and that changing environments from coniferous forest to desert played a bigger role in shaping H. kennerlyi and H. nasicus. This is all speculation of course because the timing of speciation events is not known. The relationship between H. simus and H. nasicus/kennerlyi is fairly recent and the relationship between H. kennerlyi and H. nasicus is very very recent. The difficulty is that any type of molecular clock that can be established for these species will have a huge, and i mean huge, variance in estimating the timing of events for these recent events. The variance would be in millions of years for something that probably took place within the last 15-50 thousand years. This makes it difficult to associate these events with any known biogeographic events. However, there has been some attempt on my part to look for biogeographic events that might coinside with the boundaries between H. nasicus and H. kennerlyi and I can say that there isn't much that sticks out.
Simply, the boundary between H. nasicus and H. kennerlyi may not represent the location of a biogeographic event at all. It may represent hybridization, clinal variation or secondary contact with subsequent competitive exclusion. My bets are on the latter. The distinction between H. nasicus and H. kennerlyi (i.e. the number of azygous scales and accessory azygous scales) geographically occurs abruptly and over a short distance. Competive exclusion would account for the somewhat steep clinal nature of the number of azygous scales. It is the abruptness of the change and no corresponding environmental change that convinces me that it isn't simply a cline.
Conversely, it is the gradual change in character (dorsal blotch number) with corresponding change in temperature, rainfall, and vegetation that convinced me that H.n.gloydi was part of a cline and thus not genetically or reproductively (some people make a distinction here)isolated from the H.n.nasicus population. I wasn't the first to note this. Platt in his 1969 publication demonstrated this quite nicely but did not have the overall sample size to demonstrate it properly.
Curtis Eckerman
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